T-B- severe combined immunodeficiency (SCID) is a group of rare monogenic primary immunodeficiency disorders characterized by a lack of functional peripheral T and B lymphocytes, resulting in recurrent early-onset severe respiratory viral, bacterial or fungal infections, diarrhea and failure to thrive. Hypersensitivity to ionizing radiation is a characteristic feature of some of its sub-types. The disease is Monarch Disease Ontology identifier MONDO_0017855 (T-B- severe combined immunodeficiency). Also known as: T-B- SCID.